Habitat constraints and spatial bias in seabird colony distributions

What governs the size and location of seabird colonies has long intrigued population ecologists. Previous analysis of the distribution of colonies of four European seabirds revealed a spatial bias - large colonies occurred farther apart than expected by chance alone - suggesting that intraspecific competition for food supplies during breeding may regulate colony size. Here we use computer intensive statistics to show that a similar spatial bias exists in three burrow-nesting seabirds - Cassin's anklet Ptychoramphus aleuticus, rhinoceros auklet Cerorhinca monocerata, and ancient murrelet Synthliboramphus antiquus - nesting off the Pacific coast of Canada. Local habitat constraints explain much of the existing spatial bias: large colonies cannot fit on small islands, and large islands suitable for colonies tended to occur far apart. However, a residual spatial bias still remained for ancient murrelets and Cassin's auklets (but not rhinoceros auklets) after habitat constraints are built into the analysis, for which intraspecific food competition remains a plausible explanation.